With an increase in amount of information over a network, a wavelength division multiplexing (WDM) system has been widely used. In the WDM, a plurality of optical signals are multiplexed using a plurality of wavelengths to be transmitted. Therefore, in the WDM system, the number of wavelengths to be multiplexed is increased, so that an amount of transmission is increased.
In the WDM system, all or a part of optical nodes is configured to include an optical add-drop multiplexer (OADM). The OADM can insert an optical signal into an optical transmission line in units of wavelength and can extract the optical signal from the optical transmission line in units of wavelength.
A function or apparatus which inserts an optical signal having a desired wavelength to a WDM optical signal is referred to as an optical add function (or optical add apparatus). A function or apparatus which branches an optical signal having a desired wavelength from a WDM optical signal is referred to as an optical drop function (or optical drop apparatus). Note that, an OADM which inserts and/or branches an optical signal having a desired wavelength is sometimes referred to as a reconfigurable optical ADM (ROADM).
In a network with an ROADM, three functions of color-less (no wavelength dependency), direction-less (no direction dependency), and contention-less (no contention in the same wavelength) are considered to be important.
Herein, the “color-less” denotes a configuration or function where an arbitrary wavelength is input to an arbitrary port of an OADM to be inserted into a WDM signal of an optical transmission line and an arbitrary wavelength from an arbitrary port is branched from a WDM signal of an optical transmission line to be input. Further, the “direction-less” denotes a configuration or function where an OADM is configured to include a plurality of optical transmission routes (Degree) to insert an optical signal input from an arbitrary port to an arbitrary route and to branches an optical signal from an arbitrary route to input the optical signal to the arbitrary port. Furthermore, the “contention-less” denotes a configuration or function which avoids contention between optical signals with the same wavelength in an inserting function unit and a branching function unit of the OADM.
An OADM having the above-described three configurations or functions is sometimes referred to as a CDC OADM, and an OADM having the two functions of color-less and direction-less functions is sometimes referred to as a CD OADM.    [Patent Document 1] Japanese Laid-open Patent Publication No. 2012-015726    [Patent Document 2] Japanese Laid-open Patent Publication No. 2012-114640    [Patent Document 3] Japanese Laid-open Patent Publication No. 2012-182665
By solving wavelength dependency and route dependency of a connection port of a block which adds/drops optical wavelength, the CD function can change a wavelength route in remote control without a task of changing fiber connection. Therefore, there is an advantage of reducing operating costs.
However, in the case where there are a large number of route (Degrees) of the ROADM, since contention (contention in the same wavelength) may easily occur, the contention-less function becomes important. For example, in the case where a ring network is to be configured by using the ROADM, the number of routes (Degrees) to be used is about two, and thus, contention does not become a major problem.
However, in the case where a mesh network or multi-ring is to be configured, since the number of route included in one ROADM becomes a plural number (three or four or more), the case where the same wavelength is connected to a plurality of routes is increased.
Herein, with only the CD function, since contention (contention in the same wavelength) easily occurs, a maintenance person needs to check whether or not there is contention at a site (block) to which an optical fiber is to be connected in order to connect the optical fiber. According to the contention-less function, since an optical fiber can be freely connected to an arbitrary block, it is possible to suppress maintenance and operating costs.
In addition, sometimes, a coherent transceiver is used for a transponder (TP or TRPN) which inputs and outputs a wavelength. In the case of using the coherent transceiver, in order to densely accommodate wavelength arrangement for large-capacity transmission (for example, 400 G/1 T), a technique called “grid-less” for flexibly configuring wavelength arrangement positions may be introduced.
According to the above technique, unlike a fixed bandwidth (50 GHz or 100 GHz) in the related art, the bandwidth can be configured to be variable. Therefore, the number of wavelengths which can be included in one WDM optical signal can be greatly increased. Accordingly, frequency of use of the same wavelength is also increased.
However, in order to implement a contention-less function, a circuit configuration is increased in size and is expensive in comparison with a CD function. For example, an ROADM having a CDC function needs to include a non-blocking switch block in order to implement contention-less.
Therefore, a large-scale N×M wavelength selection switch (WSS: Wavelength Selective Switch), an N×M optical cross connect switch (OXC), or the like needs to be used, or a large number of optical devices need to be combined. As a result, costs of the ROADM are high, and the size of ROADM is increased. This causes too high costs of equipment investment in the initial introduction stage of the ROADM.
Therefore, in many cases, in the initial introduction stage, a CD function which can be implemented by using a small number of inexpensive optical devices is introduced into the ROADM. However, when a large number of wavelengths are extended and routes are also extended after the operation, the case of extending the same wavelength is increased. At this time, it is very inconvenience to determine whether the wavelength can be connected by an optical fiber every time, and the operating costs are increased.
Namely, according to the extension of the number of routes or the extension of the number of added/dropped wavelengths, necessity of a CDC function as well as a contention-less function is increased.
Therefore, although a CD function is introduced initially, according to the extension of the routes or an increase in the number of extended wavelengths, a function capable of adding a CDC function is needed.
However, with respect to the CD function configuration in the current state, configuration blocks of the apparatus may be greatly changed in order to incorporate the CDC function into the OADM in the middle of the operation. Therefore, CDC function may not be extended in the OADM without influence on the optical signal during operation (in-service).